Amplifier for intercommunication system with fire and burglar alarm features



Feb. 3, 1970 E. o. RESSLER 3,

AMPLIFIER FOR INTERCOMMUNICATION SYSTEM WITH FIRE AND BURGLAR ALARMFEATURES Filed April 3, 1967 l9 LOAD (SPEAKER) Q gOMMON SWITCH POSITIONSa-BURGLAR aFIRE Io-OFF (RESET) c-FIRE ONLY ol-FIRE TEST A A -zzv SUPPLYg gm BURGLAR ALARM SWITCHES R m E L N S E s V E N R 0 D R A H E FIREALARM SWITCHES ATTORNEY.

United States Patent 01 3,493,967 Patented Feb. 3, 1970 3 493,967AMPLIFIER FOR INTERCOMMUNICATION SYSTEM WITH FIRE AND BURGLAR ALARMFEATURES Erhard 0. Ressler, West Chester, Ohio, assignor to AvcoCorporation, Cincinnati, Ohio, a corporation of Delare Filed Apr. 3,1967, Ser. No. 627,751 Int. Cl. G08b 1/08 US. Cl. 340420 Claims ABSTRACTOF THE DISCLOSURE A solid state audio amplifier of the type used insound intercommunication systems is provided with a feedback networkwhich is adapted either to make the amplifier howl with high frequencyoscillations, in the event of a fire, or provide low frequencyoscillations of the relaxation type, in the event of a burglary.

An electronic switch in the form of a bistable device controls thepotential of a point on the feedback network in order to establish thesetwo conditions of operation. A control switch establishes a response tothe sensing of both burglary and fire or fire only. It also has an oposition.

BACKGROUND OF THE INVENTION The present invention provides controlcircuitry which, when used in conjunction with an intercommunicationsystem having a sound channel, provides means for detecting suchundesired hazards as burglary or fire, and adds these additionalfunctions to the normal function of an intercommunication system.

The prior art includes many arrangements for providing an audio tone inresponse to the sensing of a fire and coupling that audio tone into theloud speakers of an intercommunication system. Such a system is shown,for example, in the United States patent to Wooten, No. 2,942,245.

The present invention provides an arrangement which furnishes an alarmin the event of either burglary or fire. In the event of fire itfurnishes high frequency oscillations in the form of a howl and in theevent of a burglary it furnishes low frequency audible sounds byrelaxation oscillations.

The present invention is an all solid state system, simple and reliablein operation, and it includes a rotary switch device operative toprovide the functions mentioned in the abstract set forth above.

DESCRIPTION OF THE DRAWINGS For a better understanding of the invention,together with other objects, advantages and capabilities thereof,reference is made to the following description of the accompanyingdrawings, in which:

FIG. 1 is a circuit schematic of an amplifier and a combined burglar andfire alarm circuit in accordance with the present invention;

FIG. 2 is a circuit schematic showing the switching functions performedwith respect to point 43 of the feed back network provided in accordancewith the present invention.

DETAIL DESCRIPTION OF THE INVENTION In accordance with the presentinvention, an audio amplifier in an intercommunication system is made tofunction as an oscillator and to set up a beep or howl in the loudspeaker load of the amplifier, in the event of the presence of a burglaror of fire, which presence is appropriately sensed.

Referring now first to the audio amplifier of FIG. 1, it comprises fourstages, the first of which is a pre-amplifier stage incorporating a PNPtype transistor 11, the second of which is an intermediate stageincorporating an NPN transistor 12, the third of which is a driver stageincorporating a PNP type transistor 13 and the fourth of which is apush-pull arranged output network comprising PNP type transistor 14 andNPN type transistor 15, connected in complementary symmetry. Thejunction of the emitters of transistors 14 and 15 is coupled by a seriescapacitor 16 to an output terminal 17. The other output terminal 18 isgrounded. To ground are connected the collector of transistor 15 and theemitters of transistors 11 and 13. A loud speaker load symbolicallyindicated by the reference numeral 19 is connected across the outputterminals 17 and 18.

A voltage feedback connection originates at the junction 20 of theemitters of transistors 14 and 15. For purposes of temperaturestabilization the direct current voltage feedback circuit comprisespoint 20, series resistor 21, series resistor 22 and series resistor 23,the junction 24 of the latter two resistors being connected to the baseof transistor 11 and resistors 21-23 forming a voltage divider betweenpoint 20 and ground. A portion of the alternating current in thefeedback path is bypassed by the series combination of resistor 25 andcapacitor 26, in shunt with 2223, some feedback being maintained forpurposes of distortion reduction.

The audio input to pre-arnplifier transistor 11 is via the inputterminals 27 and 28, terminal 27 being coupled to the base of transistor11 via capacitor 29.

A direct current reverse-bias voltage of approximately 18 volts isprovided for the collector of pre-amplifier transistor 11 via a seriesconnection of resistors 9, 30 and 31, resistor 31 being connected to a22 volt terminal 32 and resistor 9 being connected to the collector oftransistor 11.

The pre-arnplifier transistor 11 is in the common emitter configurationand its collector output is directly connected to the base of transistor12 and the collector output of transistor 12 is in turn directlyconnected to the base input of transistor 13, also in the common emitterconfiguration. Resistor 33 is the collector load of transistor 12.Capacitor 34 is an emitter bypass for transistor 12.

Current in resistor 35, in series with the collector of transistor 13,provides turn-on bias for transistors 14 and 15, resistor 35 beingconnected between their bases. Resistor 36, being large with respect toresistor 35, is the principal collector load for transistor 13.Resistors 35, 36 and 31 are in series between the collector oftransistor 13 and supply terminal 32.

There is a bootstrap coupling from output point 20 of the push-pulloutput stage (i.e. the emitters) to the junction 37 between resistors 31and 36, via capacitor 38, and the purpose of this bootstrap connectionis to provide a more elevated voltage for the collector of transistor 13than is available at the source terminal 32.

In accordance with one aspect of the invention, this amplifier is madeto oscillate by close-circuiting of a feedback network between the audiooutput terminal 17 and the collector of transistor 11. This networkcomprises, in series, a series resistor 41, series capacitor 40, andseries diode 39. The diode 39 is so arranged that its cathode has aresistive shunt connection 42 to the 22 volt supply terminal 32 whereby,when the cathode of the diode is not grounded the cathode is forwardbiased and the diode 39 then establishes a conductive path and afeedback connection between point 17 and the collector of transistor 11,which feedback connection, when closed, causes regenerative reaction andoscillation. The point at which the cathode is connected to resistor 42is referred to as 43.

When the point 43 is grounded then the feedback con nection iseffectively open-circuited and the amplifier circuit functions as anamplifier. On the other hand, when the point 43 is ungrounded then thefeedback connection 39, 40, 41 is close-circuited and the amplifierfunctions as an oscillator.

The broad function of the remaining parts fthe circuit now described isto cause point 43 to be grounded or ungrounded as desired, groundingbeing the condition desired for normal operation and a negativepotential being the condition desired when a warning or fire is to befurnished. For burglary, point 43 is at a less negative potential.

At this point it is emphasized that when point 43 is grounded theamplifier is in the upper part of FIG. -1 functions as such. Referenceis now made to FIG. '2. When point 43 is negative enough to make diode39 conductive, as when the presence of a fire is detected, then thefeedback network 41, 40, 39 functions in such a way as to make theamplifier howl at a relatively high but audible frequency. On the otherhand, when burglary is detected point 43 is placed at a less negativepotential and the feedback network then functions in such a way as tocause the generation of relaxation oscillations which are at a lowerfrequency and therefore characterize the detection of burglary. In otherwords, when a fire is detected point 43 is placed at a negativepotential. When burglary is detected point 43 is placed at a lessnegative potential. This switching of point 43 to the desired potentialis accomplished by the switching network or bistable device comprisingthe transistors 50 and 51. Resistors 70 and 67 and the collector-emittercircuit of transistor 51 are in series between high potential supplyconductor 72 and conductor 57. The emitter collector circuit oftransistor 50, resistor 68 and resistor 71 are also in circuit betweenconductor 72 and conductor 57. The operation of this switching circuitryis such that when a burglary is detected, removing the ground from 57,then conductor 52 is in effect encircuited with conductor 72 and placedat a minus potential, whereby the diode 61 becomes nonconductive and thepotential of point 43 is as determined by the voltage divider whichcomprises resistor 42 and resistor 65, disposed between conductor 72 andground, point 43 being effectively the junction of these two resistors.

On the other hand, when a fire is detected the connection between point43 and ground is simply broken. This will be more readily understood byreference to FIG. 2 in which 61 is a diode and a switch with movingcontact Z and fixed contacts X and Y symbolically represents the switchincluding the transistors 50 and 51 of FIG. 1. It will be seen from FIG.2 that when one of the burglary sensing switches 58 or 59 is open themoving contact Z moves to X, turning off diode 39 and placing point 43at a potential determined by the voltage divider 42, 65. On the otherhand, when one of the fire alarm switches 54 or 55 is open, point 43 issimply ungrounded. This discussion of FIG. 2 assumes that the switch X,Y, Z operates in such a manner that Z is normally in contact with Y andthat when either 58 or 59 is opened that Z will immediately swing overthe contact with X.

By analogy, when 58 or 59 in FIG. 1 is open, 57 departs from groundpotential and the collector of 51 immediately falls to -22 volts, sothat the analog between the bistable switching device of FIG. 1 and thesymbolic representation of FIG. 2 is satisfied.

A switch comprising three ganged sections 44A, 44B and 440, the gaugingof the rotary contacts being by a suitable mechanical expedient 45, isso arranged as to provide for four distinct conditions of operation. Inthe extreme counterclockwise position the rotary contacts areencircuited with the respective fixed contacts designated a. In the nextposition, the rotary contacts are encircuited with the fixed contactsdesignated b. In the third position, the rotary contacts are cncircuitedwith the fixed contacts designated 0, and in the fourth or extremeclockwise position with those designated d. The b contact of section 44Ais connected to point 43. The a and 0 contacts of switch section 44C areconnected to each other and point 43. The I), c and d contacts ofsection 44B are connected to each other and to the collector oftransistor 51. The rotary contacts of switches 44A and 44B are connectedto ground.

Let it now be assumed that the switch sections are in the off position.In this condition there is ground connection, from 43 via conductor 62,the b contact of section 44A, conductor 47, and conductor 48 to ground,which open-circuits and disables the feedback connection 39, 40 and 41.This establishes normal amplification of the amplifier.

Parenthetically, for reasons which will appear hereinafter, when theswitch sections are in the off position it is desired that the bistabledevice which comprises transistors 50 and 51 be conductive, i.e., thatthe collector of transistor 51 be at ground potential and thisrequirement is accomplished by the connection comprising the collectorof 51, conductor 52, contact b of switch section 44B, conductor 48 andground 60.

Let use now consider the position Bulglary-Fire condition of the switchsections 44A, 44B, and 44C. In this position it is desired that thesystem be sensitive to the presence either of a burglar or fire. Thepresence of a fire is indicated by the opening of any one of theserially arranged thermally responsive switches 54, 55, etc.,individually staged at the various locations where it is desired thatthe presence of a fire be sensed. In other words, the presence of a fireis indicated by an open circuit on line 56.

The presence of a burglar is indicated by an open circuit on line 57, byreason of the opening of any one of the switches 58 and 59 which areconnected in series and individually located at the various places wheresecurity against intrusion is desired.

It will be recalled that the bistable device which comprises theelements 50, 51 is conductive when the switch sections were in the offcondition. When the switch sections are placed in the Burglary-Fireposition this bistable device remains conductive, effectivelyshort-circuiting conductor 57 to conductor 52 so that a ground circuitis established from conductor 60, switches 59 and 58, conductor 57, theemitter-collector circuit of transistor 51, and conductor 52, diode 61,switches 54 and 55, conductor 56 and the a contact of switch section44C, conductor 62 and point 43, which point is therefore grounded sothat the amplifier functions as an amplifier. Note that the existence ofthis ground depends on: the closed state of switches 54-55 and 5859 andthe conductivity of transistor 51.

Let there now be considered the events which occur upon opening of anyone of the fire alarm switches 54 and 55. This action places point 43completely off ground because there is an opening in the circuitcomprising the elements 62, a contact of 44C, 56 and 55, 54. This actioncauses a single tone oscillation to be produced by regenerative actionof the aforementioned feedback components 39-41.

Now the tone which is characteristic of the burglar alarm is differentthan that which characterizes a fire alarm.

Let there be considered the events which occur when there is a burglaralarm, causing the opening of one of switches 58 and 59 and ungroundingconductor 57. This action causes interruption of current flow throughthe bistable circuit comprising transistors 50 and 51, whereby thecollector of 51 and the conductor 52 fall to approximately 22 volts, thepotential of the collector of 51 rendering diode 61 non-conductive sothat point 43 is effectively connected to point 64, via 62, the acontact of switch section 44C, conductor 56 and switches 54 and 55. Thepotential at point 43 is now dependent on the proportions of theresistors 42 and 65 and is such that the feedback elements 39, 40 and 41are causing blocking type oscillations to occur.

The capacitor 40 charges up in a positive direction, periodicallycutting olf diode 39 and this causes the oscillations to be of theblocking type. Once an oscillation stops, capacitor 40 dischargesthrough 43 and 32, again rendering diode 39 conductive.

The bistable is a collector to base coupled complementary pair, thecollector to base couplings being via resistors 67 and 68. Capacitor 79is connected between the emitter and base of transistor 51 for purposesof transient suppression. Appropriate base biases are provided byresistors 70 and 71.

Let use now consider the events which occur when the switch sectionsemploy the fixed contacts c in the Fire Only position. Under thiscondition contact of switch section 44B short-circuits the collector oftransistor 51 to ground. Conductor 52 being at ground potential,switches 58 and 59 are no longer effective. That is, the system does notrespond to a burglar alarm when it is set for fire only. In the absenceof a fire, 43 remains connected to ground via 62, fixed contact c of44C, 56, 55, 54, 64, 61, 52, 0 contact of 44B, conductor 48 and ground.In the event of a fire, one of switches 54 or 55 will open, ungroundingpoint 43.

The d contact-utilization or Fire Test condition of the switch sectionssimulates the conditions of fire only, on the assumption that one of thefire switches is open. That is, it open-circuits line 56. It alsoconnects the collector of 51 to ground 60.

The invention provides:

In combination:

An audio amplifier,

A resistance capacitance-feedback network 41, 40, 39 in said amplifierwhich is responsive to a first potential (ground) to cause the amplifierto operate as such, to a second potential (a negative one) to cause theamplifier to oscillate, and to a third potential (a less negative one)to cause the amplifier to relaxation oscillate, said network having acontrol terminal 43 to which said potentials are applied, and means forapplying said potentials to said control terminal comprising:

A source of potential having positive and negative terminals (32 and18),

A voltage divider comprising a first resistor 42 and normally closedfire sensing means 54, -5 and a second resistor 65 in series between theterminals of said source,

Means comprising a normally enabled diode 61 and normally closedburglary sensing means 58, 59 in series across the second resistor 65for normally placing said control terminal 43 at the first potential(ground), and

Means (the bistable device including 50, 51) responsive to the openingof the burglary sensing means for disabling said diode to place saidcontrol terminal 43 at the third potential (the less negative one),

Said control terminal being connected to the first resistor and the firesensing means (by 62, a of 44C and 56) so that when the fire sensingmeans is open the control terminal 43 goes to the second potential (themore negative one) by reason of voltage division.

I claim:

1. In combination:

an audio amplifier,

a resistance capacitance-feedback network in said amplifier which isresponsive to a first potential to cause the amplifier to operate assuch, to a second potential to cause the amplifier to oscillate, and toa third potential to cause the amplifier to relaxation oscillate, saidnetwork having a control terminal to which said potentials are applied,and means for applying said potentials to said control terminalcomprising:

a source of potential having positive and negative terminals,

a voltage divider comprising a first resistor and normally closed firesensing means and a second resistor in series between the terminals ofsaid source,

means comprising a normally enabled diode and normally closed burglarysensing means in series across the second resistor for normally placingsaid control terminal at the first potential, and

means responsive to the opening of the burglary sensing means fordisabling said diode to place said control terminal at the thirdpotential, by reason of voltage division,

said control terminal being connected to the first resistor and the firesensing means so that when the fire sensing means is open the controlterminal goes to the second potential.

2. In combination, an audio amplifier,

a source of potential having positive and negative terminals,

means in said amplifier having a control terminal which is normally at areference potential but which responds to second and third potentials tocause the amplifier to oscillate in distinct modes,

a normally closed fire sensing means,

a normally closed burglary sensing means,

means responsive to the opening of the fire sensing means for placingsaid control terminal at the second potential, and

means responsive to the opening of the burglary sensing means forplacing said control terminal at the third potential,

the means responsive to the opening of the fire sensing means comprisinga first resistor between one of said source terminals and said controlterminal,

the means responsive to the opening of the burglary sensing meanscomprising a second resistor in series with the other source terminaland said fire sensing means and said control terminal,

whereby opening of the fire sensing means places said control terminalat the potential of said one source terminal, and closing of the firesensing means places said control terminal at the potential of thejunction between the fire sensing means and said second resistor.

3. The combination in accordance with claim 1,

means for normally holding the control terminal at the referencepotential comprising a diode in series combination with said firstresistor and said fire sensing means and said burglary sensing means andsaid other source terminal, and

means for closing this series combination but responsive to opening ofthe burglary sensing means to disable the diode and to permit thecontrol terminal to assume the third potential determined by said firstand second resistors.

4. The combination in accordance with claim 3 in which thelast-mentioned means is a bistable device comprising two transistors ofwhich one has a collector-emitter circuit in series with said diode andsaid burglary sensing means, whereby the combination of said othersource terminal, which is at the reference potential, and the burglarysensing means and said collector-emitter circuit and said diode and saidfire sensing means and said first resistor normally cooperate to holdsaid control terminal at said reference potential.

5. In combination, an audio amplifier,

a source of potential having positive and negative terminals,

means in said amplifier having a control terminal which is normally at areference potential but which responds to second and thirddirect-current biasing potentials to cause the amplifier to oscillate indistinct modes,

a normally closed fire sensing means,

a normally closed burglary sensing means,

means responsive to the opening of the fire sensing means for placingsaid control terminal at the second direct-current potential and meansresponsive to the opening of the burglary sensing means for placing saidcontrol terminal at the third direct-current potential,

the means responsive to the opening of the fire sensing means comprisinga first resistor between one of said source terminals and said controlterminal,

and the means responsive to the opening of the burglary sensing meanscomprising a second resistor in series with the other source terminaland said fire sensing means and said control terminal,

8 whereby opening of the fire sensing means places said control terminalat the potential of said one source terminal, and closing of the firesensing means places said control terminal at the potential of thejunction 5 between the fire sensing means and said second re- JOHN W.CALDWELL, Primary Examiner P. PALAN, Assistant Examiner US. Cl. X.R.

